Download Parallel Processing

By Thomas L. Dehs
Structured Scientific Software
1509 Queen Ave. SW
Albany, OR 97321
Data abstraction
and dynamic
memory allocation
are two areas
where Modula-2
and Ada shine.
easily, and move them about the screen.
No use wasting memory after we've
thrown away a block, so we'll allocate
and deallocate memory dynamically.
(Since we want our program to be portable, we won't be able to move blocks
from display memory, since the locations
may be hardware dependent.)
We have a DEFINITION MODULE of
procedures, so we can start building the
low-level tools we need to implement
this set of procedures.
The first of these tools is a data structure for storing a screen block. This
structure must know where the block
came from, its size, where its characters
lie, and their attributes.
In The Low Levels
The most primitive unit in this data
structure is the character and its attribute
- a pair of bytes.
The Modula 2 module called SYSTEM
has a data type BYTE with which we'll
set up a two-byte (character and attribute) data structure.
TYPE CA = ARRAY[O .. l] OF ~YTE;
(continued next page)
Figure 3 - ScreenBlocks Implementation Module
IMPLEMENTATION MODULE ScreenBlocks;
(* This module is system specific. This version is written for
the IBM-PC and clones using MS-DOS. *)
FROM Storage IMPORT ALLOCATE,DEALLOCATE,Available;
FROM Strings IMPORT Assign,CompareStr;
FROM SYSTEM IMPORT AX,BX,CX,DX,SETREG,GETREG,CODE,
SWI,TSIZE,BYTE,WORD;
CONST
TYPE
rows
25;
cols
ao;
NumBlocks = 10;
PUSHBP
0055H;
POPBP
005DH;
INT10
OOlOH;
READCH
OaOOH;
WRITECH
0900H;
GETMODE
OFOOH;
CURSOR
0200H;
ROWINC
OlOOH;
NAMELENGTH = 24;
CA
= ARRAY [0 .. 1]
OF BYTE;
(* Contains char value and
attribute. *)
(* CA[O] is the character and CA[l] is the attribute. *)
= ARRAY[O .. cols - 1] OF CA; (* Each line of the
ao col display. *)
R
RowPointer = POINTER TO R;
BlockType = ARRAY[O .. rows - 1] OF RowPointer;
ScreenBlock
RECORD
Handle: ARRAY[O .. NAMELENGTH] OF CHAR;
FirstRow,LastRow,FirstCol,LastCol : CARDINAL;
Row : BlockType;
END;
BlockPointer = POINTER TO ScreenBlock;
BlockArray = ARRAY[O .. NumBlocks - 1] OF BlockPointer;
=
VAR BlockSpace : BlockArray;
PROCEDURE CutBlock(FirstRow,LastRow,FirstCol,LastCol
Handle: ARRAY OF CHAR ; VAR done
CARDINAL;
BOOLEAN);
VAR I,J,K,NumCols,Position : CARDINAL;
A : BlockPointer;
MODE,PAGE,TEMP : WORD;
BEGIN
done := FALSE;
(* Test for legitimate input. *)
IF «(FirstRow <= LastRow) AND (FirstCol <= LastCol»
AND «LastRow < rows) AND (LastCol < cols») THEN
MICRO CORNUCOPIA, #38, Nov-Dec 1987
49